Acute liver failure (ALF) is a devastating condition with high mortality (60-80%), and liver transplantation is the only life-saving option for ALF due to the lack of early diagnostic indicator(s) and effective pharmacologic treatment. Incidence of drug-induced (acetaminophen [APAP] being most common) hepatotoxicity is very high in general and in particular VA populations, and accounts for about 15% of the liver transplants. Therefore, precise knowledge of the mechanisms and diagnostic indicators of ALF become critical to develop therapeutic interventions since the failing liver may revive if given proper treatment in a timely manner. There is compelling evidence for an association between increased endotoxin (lipopolysaccharide, LPS) and ALF. Alcoholism, incidence of which is high among veterans, pre-existing undiagnosed liver disease or even simple fever all increase LPS and can put these subjects at high risk of liver failure due to APAP. Indeed, chronic alcohol intake is more associated with APAP hepatotoxicity than fasting. Although APAP overdose increases probability of ALF, serious liver damage at therapeutic doses has also been observed, which may be due to preexisting condition such as elevated LPS. Mechanisms of such predisposition are poorly understood. Our research shows that the perisinusoidal hepatic stellate cells (HSCs) strongly react to very low concentrations of LPS and regulate hepatocyte survival via soluble mediators, and further indicates that HSCs regulate function of liver-resident macrophages, Kupffer cells. LPS-induced modest acute liver injury is ameliorated in a novel HSC-depleted mouse developed in our laboratory. However, excessive damage occurred when HSC-sufficient LPS-treated fed mice were challenged with otherwise tolerated dose of APAP. In contrast, HSC-depleted mice were protected from LPS/APAP-injury. This observation is highly significant as high dose APAP-induced liver injury is investigated almost exclusively in starved mice; starvation depletes hepatocytes of ATP and GSH rendering them vulnerable to excessive death. Our preliminary data indicate that IFN? released by LPS-stimulated HSCs is a critical mediator that induces activation of interferon-regulatory factor 1 (IRF1) in hepatocytes and conditions them to subsequent APAP-induced death. We also found that serum levels of a protein named ?Augmenter of Liver Regeneration (ALR)? are elevated prior to ALT or inflammatory cytokines in various models of acute liver injury. ALR is constitutively produced and secreted by hepatocytes and its mitochondrial presence is essential for their survival. Our in vitro and in vivo experiments show that LPS/HSC + APAP elicits greater ALR release by hepatocytes. Based on these findings, we propose to test the hypothesis that HSCs, directly and by modulating Kupffer cell responses, orchestrate liver failure in LPS-preconditioned mice by otherwise innocuous dose of APAP via the IFN?/IRF1 axis. We further hypothesize that serum ALR can be a reliable diagnostic biomarker for ongoing hepatocellular stress or injury. Our AIMS are-- AIM 1: To investigate the role of HSCs in LPS/APAP-induced liver injury and that ALR is an early reliable serum biomarker for ALF. AIM 2: To delineate molecular mechanisms of APAP-induced hepatocyte death by LPS/HSC in vitro. AIM 3: To investigate the role of HSCs in modulating characteristics of Kupffer cells. The outcome of this investigation is expected to provide much improved understanding of the mechanisms of APAP-induced liver injury, with implications in alcoholism and other conditions that predispose the liver to excessive damage. We also expect to identify ALR as an early biomarker of APAP-induced injury. This knowledge may be useful in developing strategies for timely therapeutic intervention for ALF.